This disclosure relates to gas turbine engine fuel systems, and more particularly to fuel pump health detection of a gas turbine engine using a variable speed start system.
Aircraft gas turbine engines typically receive pressurized fuel from gear-type fuel pumps. Gear pumps perform over a wide operational speed range while providing needed fuel flows and pressures for various engine performance functions.
Gear pumps often comprise two coupled gears of similar configuration and size that mesh with each other inside an enclosed gear housing. A drive gear may be connected rigidly to a drive shaft. As the drive gear rotates, it meshes with a driven gear, thus rotating the driven gear. As the gears rotate within the housing, fluid is transferred from an inlet to an outlet of the gear pump. Typically, the drive gear carries the full load of the gear pump drive or input shaft. The two gears may operate at high loads and high pressures, which may stress the gear teeth.
The volume of fluid pumped through the gear pump may partially depend on the geometry of the tooth (e.g., depth, profile, etc.), the tooth count, and the width of the gear. Larger volumetric output may be achieved when lower gear tooth counts with large working tooth depths and face width are used. Alternatively, higher volumetric output may be achieved with higher rotational speed of the pump. As the gears rotate, individual parcels of fluid are released between the teeth to the outlet. A common problem with more traditional gear pumps operating at high rotational speeds is cavitation erosion of the surfaces of the gear teeth. Cavitation erosion results in pitting of surfaces of the gear teeth that may eventually result in degraded pump volumetric capacity and affect pump operability and durability.
Fuel flow performance erosion is typically not detected until the start flow is not adequate to meet the required flow for engine starter assisted light-off speed (typically 10-20% engine speed) and acceleration of the engine. This can lead to a delay and/or cancellation of the flight. Prior to the inability to start on the ground with starter assist, the fuel pump would likely have been severely degraded for some prior period and would be unable to perform a windmill relight at altitude at aircraft speeds associated with engine windmill speeds without starter assist (typically 6-12% engine speed).
This undetected condition could lead to an inability to relight at critical points in the flight envelope without starter assist and is undesirable. It is critical for fuel pumps to be able to build up enough fuel pressure and flow to restart the engine in flight during windmilling conditions following an in-flight shutdown event. The windmilling condition for engine re-start is typically very low (8-10% of ground idle speed) compared to a nominal ground-based engine starting speed.
In addition to ground start and windmill relight potential issues, there is potential for inadequate pump fuel flow to make adequate takeoff thrust. Inadequate fuel flow may be detected during takeoff conditions and could lead to an aborted takeoff. To safeguard against this, fuel pumps are typically removed for maintenance at some periodic interval.